Update the TTL field in forwarded packets

Closes: #1085

2 files changed, 17 insertions, 0 deletions

diff --git a/pkg/tcpip/header/ipv4.go b/pkg/tcpip/header/ipv4.go
index 76839eb92..d8111a439 100644
--- a/pkg/tcpip/header/ipv4.go
+++ b/pkg/tcpip/header/ipv4.go
@@ -248,6 +248,18 @@ func (b IPv4) SetDestinationAddress(addr tcpip.Address) {
 	copy(b[dstAddr:dstAddr+IPv4AddressSize], addr)
 }
 
+func (b IPv4) UpdateTTL(n uint8) {
+	var sum uint32
+	var old uint16
+
+	old = binary.BigEndian.Uint16(b[ttl:])
+	b[ttl] -= n
+	sum = uint32(old) + uint32(^binary.BigEndian.Uint16(b[ttl:]) & 0xffff)
+	sum = sum + uint32(b.Checksum())
+	sum = (sum & 0xffff) + (sum>>16)
+	b.SetChecksum(uint16(sum + (sum>>16)))
+}
+
 // CalculateChecksum calculates the checksum of the ipv4 header.
 func (b IPv4) CalculateChecksum() uint16 {
 	return Checksum(b[:b.HeaderLength()], 0)
diff --git a/pkg/tcpip/header/ipv6.go b/pkg/tcpip/header/ipv6.go
index 76e88e9b3..ff1aa937e 100644
--- a/pkg/tcpip/header/ipv6.go
+++ b/pkg/tcpip/header/ipv6.go
@@ -226,6 +226,11 @@ func (b IPv6) SetNextHeader(v uint8) {
 	b[nextHdr] = v
 }
 
+// SetHopLimit sets the value of the "hop limit" field of the ipv6 header.
+func (b IPv6) SetHopLimit(v uint8) {
+	b[hopLimit] = v
+}
+
 // SetChecksum implements Network.SetChecksum. Given that IPv6 doesn't have a
 // checksum, it is empty.
 func (IPv6) SetChecksum(uint16) {